A DNA chip is a device in which 10 to 105 types of DNA nucleic acid strands serving as probes are immobilized on a slide glass or silicon substrate several cm square. To analyze a sample using a DNA chip, first, the contained nucleic acids are labeled by a fluorescent dye, a radioisotope, or the like, and then made to react with the probes on the chip. If the nucleic acids in the sample contain a nucleic acid complementary to a probe on the chip, hybridization occurs. Each probe immobilized on the chip has a known sequence and immobilization position. Hence, specifying the position on the chip where a signal derived from the label is obtained enables to determine the sequence of the nucleic acid contained in the sample (non-patent literature 1). The DNA chip is a device (testing tool) of enormous usefulness in analyzing a number of genes at once for one specimen. Normally, one chip is used for one specimen. To check the expression level difference between two specimens, one chip is used for two specimens.
On the other hand, for example, in the field of infectious diseases, the number of samples may be large, although the number of genes to be checked is small. However, since it is conventionally necessary to use a lot of chips in accordance with the number of samples, the test cost including the chip, labor, and time is high.